1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method that have a density adjustment function.
2. Description of the Related Art
In order to realize preferable gradation reproduction, some image processing apparatuses such as electrophotographic printers apply different gradation targets to objects such as images, graphics and texts. A gradation target refers to target gradation properties of an image; a gradation target mentioned herein represents a function and a correction table for correcting an output image signal so as to coordinate an output image signal value with target density at which an image is formed in correspondence with the signal value. For example, a gradation target with a concave curve (that is to say, conversion for increasing intermediate density) is applied to texts for increased readability, and a gradation target with a convex curve (that is to say, conversion for reducing intermediate density) is applied to images for gradation properties.
Also, some electrophotographic printers have a function of adjusting toner density characteristics for the case where actual density characteristics have deviated from ideal density characteristics due to temporal changes and durability, and for the case where a user wishes to switch to desired density characteristics. A density adjustment function of a printer may automatically coordinate density with a gradation target that has been determined in advance, or adjust density through manual operation. In the case of density adjustment through manual operation, an inclination of a density characteristic for each of cyan, magenta, yellow and black (hereinafter noted as C, M, Y and K, respectively) is changed, or an entire density range of each of C, M, Y and K is divided into low-, middle- and high-density ranges so as to enable independent level adjustment on a per-density range basis (for example, Japanese Patent Laid-Open No. 07-221986). For example, a technique of Japanese Patent Laid-Open No. 07-221986 is advantageous in that, as density can be finely adjusted on a per-color basis and on a per-density range basis, a user can adjust coloration with high flexibility (hereinafter referred to as a “density fine adjustment” function). A printer that applies different gradation targets to different objects, as has been described above, conducts printing by applying density conversion of the density adjustment function to image data, and further applying gradation targets, screens, and the like for the respective objects to the result of the adjustment.
However, a problem with the foregoing case where different gradation targets are applied to different objects is that the density fine adjustment function uses different density adjustment regions for different objects. In other words, there are cases where appropriate adjustment is made for one object while unintended adjustment is made for another object.
Density fine adjustment will now be described with reference to FIG. 6. It should be noted that, for the sake of simplicity, the following description focuses on density fine adjustment for K only. It will be assumed that gradation targets 603, 604 and 605 are applied to images, graphics and text objects, respectively (a linear line 610 is drawn for comparison). If density fine adjustment is not performed, the gradation targets 603, 604 and 605 serve as output density characteristics as-is. Here, an output density range is trisected as indicated by output density 611 such that the output density ranges look uniform if the density fine adjustment is performed, and based on reverse calculation using the gradation target 603, input density ranges corresponding to the output density ranges are considered as adjusted density ranges. In the present example, output density characteristics obtained when the gradation target 603 for images is used are trisected so as to set mutual adjusted density ranges of the density fine adjustment for all objects. That is to say, an input low-density range of the density fine adjustment that has been set based on an output low-density range for images is a low-density range 609. The adjustment is performed by using this low-density range 609 mutually for graphics and text objects. If an adjustment value of −8 is set for the low-density range via a UI 601 for the density fine adjustment, an adjustment table 602 corresponding to the adjustment value (−8) for the low-density range 609 is internally generated. By synthesizing this adjustment table 602 and the gradation targets of respective object types, output density characteristics 606, 607 and 608 are obtained for images, graphics and texts, respectively. With regard to images, the low-density range in the final output density matches the density range adjusted by the density fine adjustment. However, with regard to graphics, the adjustment is performed further on the middle-density range in the final output density, and with regard to texts, the adjustment is performed further on the high-density range in the final output density. In other words, even though the user has intended to lighten only a low-density object, an object of middle or higher density also looks lighter with regard to graphics and texts.